1. Field of the Invention
The present invention relates to an endoscope mounted with an image pickup device configured to output a digital signal as an image pickup signal.
2. Description of the Related Art
Conventionally, an endoscope provided with an image pickup device has been widely used in a medical field and an industrial field. Further, a technique of configuring an endoscope system in which a signal processing apparatus referred to as a processor and attachably and detachably connected to an endoscope is responsible for various signal processing related to the endoscope is also known.
Further, recently, there is a tendency that, in an endoscope adopting a CCD image sensor as an image pickup device, the number of pixels of the CCD image sensor increases. In an endoscope of this kind, further faster processing is required than conventionally required as the number of pixels increases.
In an endoscope of this kind, it is difficult to appropriately transmit a signal output from the CCD image sensor arranged at a distal end of an insertion portion of the endoscope because of reasons that a cable connecting the image pickup device arranged on a distal end portion and the processor is relatively long, that a diameter of the cable is restricted in order to realize reduction in a diameter of the insertion portion of the endoscope, and so on.
In view of the above situation, a system has been also disclosed recently in which CDS processing (correlated double sampling) is performed for an image pickup signal of the CCD image sensor on the distal end portion of the endoscope to transmit the image pickup signal via the cable after reducing a frequency band of the signal, while the frequency deterioration in the cable is corrected by a predetermined analog circuit (an enhancement circuit or the like) arranged in the receiving-side processor or the like (see Japanese Patent Application Laid-Open Publication No. 2012-115531).
Note that, though the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2012-115531 is such that corrects a frequency component, a technique of digitally correcting a frequency-deteriorated component is also known.
On the other hand, recently, an endoscope adopting a CMOS image sensor as an image pickup device has been also proposed.
A CMOS image sensor of this kind is provided with an A/D converting portion together with an image pickup portion in its sensor chip. An output signal as a sensor is a digital signal.
Here, from a viewpoint of frequency characteristics, transmission of a digital signal is not easily influenced by physical characteristics of a cable itself in comparison with transmission of an analog signal. Therefore, since an endoscope adopting a CMOS image sensor of this kind outputs a sensor output signal as a digital signal, image quality is influenced little by signal deterioration even in the case of signal transmission with a relatively long cable, in comparison with an endoscope adopting a CCD image sensor configured to output a sensor output signal as an analog signal.